A Midland police authority has unanimously rejected the Home Secretary's call for its force to volunteer to merge with three other constabularies.
West Mercia Police Authority said it remained committed to a strategic service dedicated to serving Herefordshire, Shropshire and Worcestershire.
Paul Deneen, chair of the police authority, said he believed there were too many unanswered questions about the formation of a regional "superforce" combining the West Midlands, Staffordshire, Warwickshire and West Mercia forces.
The merger, which has been recommended by the police inspection service, would create the largest police force in the country outside London.
Mr Deneen said: "As custodians of a top-performing police force and as chairman of the authority I have huge concerns about the impact upon performance delivery and the disruption and distraction that any possible merger would cause."
Figures from the Home Office indicated the set up costs for the new force could total £52 million, he said.
West Mercia Chief Constable Paul West also voiced fears about the wisdom of the Government's proposals.
He said: "To contemplate a £1 billion merger with no clarity around set-up costs or council tax impact, and with so many other significant issues not yet addressed in any real detail by the Home Office and central Government, would seem to me to be extremely unwise, to put it mildly."
Charles Clarke, the Home Secretary, has told authorities and forces they have until February 24 to decide on a voluntary merger. If they refuse, there will be a further four-month consultation.
Mr Clarke can ultimately force the merger through in Parliament.